The research on different solar-to-electric energy conversion devices made of materials alternative to silicon has been boosted in recent years for a number of reasons such as the search for lower cost processes or added value features, such as transparency. Among them, one of the devices that have shown higher efficiency is dye-sensitized solar cells (DSSC), also known as Grätzel cells, U.S. Pat. No. 5,084,365. The DSSC combine a solid wide band gap semiconductor with a liquid ionic conductor. The former usually consists of one electrode made of a layer of a few micrometers of titanium dioxide nanocrystals (nc-TiO2, average crystal size around 20 nm), on whose surface a dye, typically a Ruthenium polypyridyl complex, is adsorbed. This nanocrystalline film is deposited onto a conductive, transparent substrate, typically indium tin oxide (ITO) or fluorinated SnO2, and soaked with a redox electrolyte, typically containing I−/I3− ion pairs. This electrolyte is also in contact with a colloidal platinum catalyst coated counter-electrode. Sunlight is harvested by the dye producing photo-excited electrons that are injected into the conduction band of the nanocrystalline semiconductor network, and then into the conducting substrate. At the same time the redox electrolyte reduces the oxidized dye and transports the electron acceptors species (I3−) to the counter-electrode. A record value of power conversion efficiency of 11% has been reported, although good quality cells typically provide between 5% and 8%.
In order to improve the durability of the cell, different attempts to substitute the liquid electrolyte for solid state hole conductors, such as polymers, or ionic liquids have been performed. Although stability is improved, lower values of efficiency are achieved.
Current efficiencies of the different types of Grätzel cells are still low compared to silicon based devices, which have an average power conversion efficiency of 15%, there is no doubt that they have a great potential for different reasons. First, although the efficiency is lower, there is a need for alternative materials to silicon that can be used for photovoltaic applications. The Grätzel cells can be made transparent, which implies they can be used as coatings on windows. The cells also have a potential to be made flexible, which would simplify integration on different types of surfaces. Finally, they are usually made of less expensive materials than silicon, and there is a wide variety of compounds (semiconductors, dyes, electrolytes) that can be used to build the cells.
Many different modifications of the originally proposed cell have been made in order to improve its performance, most of them based on the use of different semiconductors, dyes or ionic conductor, or on alterations of its nanostructure. In many cases, however, a change of cell constituents gives rise to an improvement of the short circuit current, but causes at the same time a decrease of the open circuit voltage or vice versa. This is due to the extreme sensibility of the charge transport and recombination dynamics to any alteration of the nature of the interfaces present in the cell.
One way to enhance the cell efficiency without affecting the delicate kinetic balance between charge separation and recombination is to modify the optical design of the cell in order to improve its light harvesting efficiency (LHE) or absorbance. This approach relies basically on an increase of the optical path resulting from the scattering of non-absorbed light by a layer of disordered particles of large size (on the order of the targeted wavelengths) placed behind the absorbing electrode. Unfortunately, some of the most successful approaches developed for silicon photovoltaic devices to improve the LHE, which are based on the implementation of highly reflecting distributed Bragg reflectors, surface gratings, or a combination of both, cannot be easily realized for liquid-semiconductor hetero junction cells due to the following reasons:                1. The need of electrical contact between the electrolyte and the sensitized semiconductor slab in the latter forces any potential back reflector to be porous to allow a proper flow of the liquid conductor through it.        2. The processing of these cells involves deposition of solid layers from colloidal suspensions, which complicates the implementation of optical quality components within the device.        
A potential solution to this problem has very recently been proposed through the implementation of highly porous photonic crystals in the solar cell structure (T. Mallouk et al. “Standing wave enhancement of red absorbance and photocurrent in dye-sensitized titanium dioxide photoelectrodes coupled to photonic crystals” J. Amer. Chem. Soc. 2003, 125, 6306; A. Mihi, H. Miguez, “Origin of Light Harvesting Efficiency Enhancement in Photonic Cristal Based Dye-Sensitized Solar Cells”, J. Phys. Chem. B. 2005, 109, 15968). A photonic crystal is primarily classified depending on the number of spatial dimensions along which there exists a periodic modulation of the refractive index, then being divided in one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) photonic crystals. One of their most representative features is their ability to diffract light.
In WO 2008102046 is disclosed a multilayer structure made of nanoparticles behaving as a one-dimensional photonic crystal for use in optical chemical sensing devices or frequency selective filters.
Coupling of a generic porous dielectric mirror in dye-sensitized solar cells is shown in the publication by A. Mihi et al. “Origin of light harvesting efficiency enhancement in photonic crystal based dye-sensitized solar cells”, J. Phys. Chem. B 2005 109, 15968.